Butterfly valves have many advantages over other types of valves in fluid flow regulation, the most significant of these being low cost of manufacture, quickness of opening and low restriction to fluid flow when fully opened. However, these valves generally have been limited to low pressure applications because of the difficulty of sealing tightly at high pressures.
In some prior art valves, the seals comprise a seat member located in a groove or grooves in the valve body. In high pressure or velocity applications, many of these seats exhibit a tendency to become dislodged from the grooves when the valves are opened, thereby rendering the valves inoperative. To prevent such seal blow-out, some valves employ elaborate groove arrangements and seat configurations, but that approach increases the initial cost of the valve, raise the cost for repair or replacement of seals, and increases the possibility of seat damage and misalignment. In several types of valves, the contact between the disc and seat must be made so tight (to prevent leakage) that closing the valve, especially large size valves, requires a large torque. In some prior art valves, having seals utilizing a groove and an elastomeric seat, the seat can cold flow under the sealing pressures employed, thereby decreasing seal life time.
There are in existence a number of butterfly valve constructions employing so-called "fluid pressure" seals. Typically in such constructions, a resilient seating member is disposed in annular relation to the fluid passage of the valve, to encircle it in such a position as to engage the periphery of a valve disc which is journalled within the passage for movement between open and closed positions. To augment the seal provided by the natural resilience of the seating member against the valve disc when the latter is in the closed position, there is generally provided a pressure space behind the seat and some means of fluid communication between the pressure space and the high pressure side of the valve. When the valve is closed, fluid under pressure is directed to the pressure space to exert an inwardly directed pressure against the seat, urging the latter into firm engagement with the periphery of the disc.
Typical also of such prior art seals are those in which a seat member is retained in a generally annular recess in the valve body and wherein the fluid pressure differential applied across the valve in a closed position forces the seat to deflect into a "corner" formed by a wall of the annular valve body recess and the peripheral sealing surface of the valve disc.
Another approach has been to provide a valve seat ring which is of lesser inside diameter than the sealing surface of the valve closure element and which is partially restrained against radial stretching by a resilient metal hoop, whereby an interference fit is achieved between the seat and disc, resulting in compression of the seat and stretching of the hoop upon closing the disc. Yet another prior art approach has been to provide a seat member retained in a valve body recess and a rigid annular projection on the downstream side of the body recess, which projection functions as a fulcrum to limit flexure under the influence of fluid pressure, to improve sealing effectiveness.
Some of these prior art valves employ a seat ring which has a "V"-shaped groove in its rear surface, in which is located a back-up ring. Both rings are, in turn, located in a "T"-shaped slot in the valve body. Upstream fluid under pressure enters the slot and moves the back-up ring along the groove to thereby force the seat ring forward into tighter engagement with the disc. This type of seal assembly has several inherent disadvantages. For example, the downstream section of the seat ring may deflect under the fluid pressure, resulting in possible back-up ring extrusion and damage. In addition, installation of the back-up ring and seat ring into the slot usually is difficult, requiring special techniques and frequently causing seal misalignment. Furthermore, these seals usually are not bi-directional; that is they seal better when the upstream side of the seal is on a particular side of the valve disc or stem.